1. Field of the Invention
The present invention relates to an electric braking device, and more specifically, an electric braking device that actuates a piston in the axial direction by the driving force of a motor to produce brake fluid pressure.
2. Description of the Related Art
A vehicular braking device is known that includes a master cylinder to which a braking operation by an operator is input and a slave cylinder which is an electric braking device and which produces brake fluid pressure based on an electric signal in accordance with the braking operation (see, for example, JP 2008-143419 A).
According to the slave cylinder of this vehicular braking device, upon driving of the motor, a piston is forwarded through a gear mechanism and a ball screw mechanism having a nut and a screw engaged with the nut. That is, the nut receives the rotational driving force by the motor, and the screw engaged with the nut moves in the axial direction to push the piston, thereby moving the piston forward. Accordingly, brake fluid pressure is produced in the fluid pressure chamber formed ahead of the piston, and the brake fluid pressure is output to the brake lines via ports.
According to the above-explained slave cylinder, however, the worn-out of the contact part between the screw and the piston affects the abutting condition of the screw to the piston, and the piston pushed by the screw may be fallen (tilted). Such fall-down of the piston results in the worn-out and damage of the piston and a seal member attached to the piston, etc.
The present invention has been made in view of such a circumstance, and it is an object of the present invention to provide an electric braking device that ensures a stable abutment of a screw to a piston.